With the development of wireless multimedia services, demands of people for high data rate and user experience are increasing, so as to raise higher requirements for system capacity and coverage of a traditional cellular network. In addition, the popularization of applications such as social networking, near field data sharing and local advertising may enable demands of people, for knowing about people or things of interest nearby and communicating therewith, to gradually increase. A related cell-based cellular network has obvious limitations to high data rate and supporting of proximity services. Under this demand background, a Device-to-Device (D2D) technology representative of a new development direction of future communication technologies emerges. Application of the D2D technology may alleviate burdens on the cellular network, reduce the power consumption of a battery of User Equipment (UE), increase the data rate, improve the robustness of network infrastructure, and well meet requirements for the foregoing high data rate services and proximity services.
The D2D technology may work at a licensed band or an unlicensed band. The D2D technology may allow multiple pieces of UE (i.e., D2D UE) supporting a D2D function to perform direct discovery/direct communication in the presence of network infrastructure or in the absence of network infrastructure. There are mainly three D2D application scenarios as follows:
1) UE1 and UE2 may perform data interaction under the coverage of a cellular network, and user plane data does not pass through network infrastructure, as shown in Mode 1 in FIG. 1.
2) UE in a weakly covering/coverless area may perform relayed transmission, as shown in Mode 2 in FIG. 1. For example, UE4 with poor signal quality may be allowed to communicate with a network via proximal UE3 covered by the network. This mode is beneficial for coverage expansion and capacity improvement of an operator.
3) When an earthquake or an emergency occurs and a cellular network cannot normally work, inter-device direct communication may be allowed, as shown in Mode 3 in FIG. 1. For example, one-hop or multi-hop data communication may be performed over control planes and user planes among UE5, UE6 and UE7 without network infrastructure.
The D2D technology may include a D2D discovery technology and a D2D communication technology.
1) The D2D discovery technology refers to a technology for judging/determining whether two or more pieces of D2D UE are proximal to each other (e.g., within a range capable of performing D2D direct communication) or judging/determining whether a first UE is proximal to a second UE.
2) The D2D communication technology refers to a technology for direct communication of some or all communication data between pieces of D2D UE without network infrastructures.
In application scenarios as shown in FIG. 2 and FIG. 3, D2D UE may serve as a relay node, such that remote D2D UE at the coverage edge of a cellular network or out of coverage may perform cellular communication with a network through the relay node UE, and D2D UEs may perform D2D communication between each other through the relay node UE. The D2D UE may move at a coverage edge of a cell, and in this case, the D2D UE may need to switch (i.e., change) the access node of the D2D UE to access the network. For example, after moving into cell coverage, a D2D UE relayed by a relay UE may access the network through a base station (which may be embodied as an evolved Node B (eNB)). After moving out of cell coverage, a UE which accesses the network through a base station within the cell coverage may access the network through a relay UE. However, a method for switching an access node of D2D UE is not provided, thus influencing the continuity of communication services of UE at the coverage edge of a cell.